1. Field of the Invention
The present invention relates to a driver for a liquid crystal device, and more particularly to a driver for a liquid crystal device that is used with multiple common drivers for comb-like driving in cascade connection.
2. Description of Related Art
Mobile devices such as cellular phones have been widely used in recent years. Liquid crystal display (hereinafter called LCD) panels for such mobile devices, in particular cellular phones, employ simple matrix display, active matrix display, or other technologies. Simple matrix display technology turns pixels on with electrodes placed lengthwise and crosswise of a display, while active matrix display technology turns each element forming a pixel on and off.
More specifically, active matrix display technology includes thin-film transistor (TFT) display technology, which uses transistors incorporated in each pixel, and thin-film diode (TFD) display technology, which uses diodes incorporated in each pixel. Among other things, TFD display technology provides as wide a range of contrast and colors as TFT display technology for representing moving images and natural colors with low power consumption. Thus TFD display technology is expected to be widely used for cellular phones and other devices in the near future.
When an LCD panel utilizing one of the above-mentioned display technologies is mounted on a mobile device, particularly on a cellular phone, the size (in particular, the width) of its outer case is almost predetermined. Therefore, if a segment driver (or an X driver, hereinafter referred to as a SEG driver) that is coupled to a segment electrode of the LCD panel and a common driver (or a Y driver, hereinafter referred to as a COM driver) that is coupled to a common electrode are put together with the LCD panel in the case, the SEG driver is placed below the LCD panel, while the COM driver is placed to the left or right of the LCD panel. As a result, the display of the LCD panel is placed to the left or right from the center of the case.
To solve this problem, the COM driver that is originally placed to the left or right of the LCD panel is usually divided into two to be placed at both sides of the SEG driver (that is, placed to the left and right of the SEG driver) below the LCD panel. Thus the SEG and COM drivers for driving the LCD panel are placed below the LCD panel, while no driver is placed on the other three sides of the LCD panel. Consequently, the LCD panel is placed on the center of the case without tilting to the left or right.
For driving such an LCD panel whose COM driver is divided into two, the following two methods can be used. One is, as shown in FIG. 10, coupling a first COM driver Y1 and a second COM driver Y2 in cascade connection, sending input data from a SEG driver X to the first COM driver Y1, and thereby driving each shift register circuit in the first and second COM drivers Y1 and Y2 sequentially so as to drive an LCD panel line by line from the top to the bottom. The other is, as shown in FIG. 11, scanning the LCD panel alternately from the right and left in a comb-like manner. Here, the latter is defined as comb-like driving.
In this comb-like driving method, the first and second COM drivers Y1 and Y2 conduct a line scan alternately from the right and left on an LCD panel. The same input data are simultaneously input from the SEG driver X to the first and second COM drivers Y1 and Y2. Moreover, by providing data that define scanning of the second COM driver Y2 follows scanning of the first COM driver Y1, each of the first and second COM drivers Y1 and Y2 alternately drives scanning lines sequentially from the top to the bottom as shown in FIG. 11 (starting from line one to lines two, three, four, etc.).
The first and second COM drivers Y1 and Y2 for comb-like driving that drive an LCD panel in a comb-like manner may be coupled in cascade connection for consecutive driving as shown in FIG. 12. This enables the first and second COM drivers Y1 and Y2 for comb-like driving to operate in cascade connection, which can enhance their utility as a driver integrated circuit on the whole.
However, multiple common drivers for comb-like driving in cascade connection involve the following problem. The problem is described below with reference to FIGS. 13 to 15.
FIG. 13 shows the first and second COM drivers Y1 and Y2 for comb-like driving in cascade connection. FIG. 14 shows input and output data, a clock signal, and shift register outputs O1 to O60 of the first COM driver Y1, while FIG. 15 shows input and output data, a clock signal, and shift register outputs O1′ to O60′ of the second COM driver Y2. It should be noted that external output data DYO (A) from the first COM driver Y1 is input data DYI (B) for the second COM driver Y2, and the timing of the external output data DYO (A) shown in FIG. 14 is identical to the timing of the input data DYI (B) shown in FIG. 15. Although FIGS. 14 and 15 should be put together in a drawing in chronological order, they are shown separately due to space limitations. Here, the first and second COM drivers Y1 and Y2 each include a built-in shift register circuit having sixty flip-flops. The drawings show an example of the common line of each driver with sixty outputs.
Each of the COM drivers for comb-like driving produces a clock (hereinafter called an internal signal XINH) for interlaced driving by halving a period of a basic clock (hereinafter called an external signal YSCL) provided from outside for driving the drivers. Then the drivers reduce the external signal YSCL by one period with the internal signal XINH so as to produce a reduced clock (hereinafter called an internal signal YSCL), which enables the drivers to conduct comb-like driving.
In the first COM driver Y1 shown in FIGS. 13 and 14, DYI (A) represents input data (triggering the first COM driver Y1 and corresponding to one period of the external signal YSCL) that is input to the first COM driver Y1 from a SEG driver not shown in the drawings, and DYO (A) represents output data (showing an end of operations of a period of the shift register) of the first COM driver Y1. The output data DYO (A) from the first COM driver Y1 are sent to the second COM driver Y2, so as to serve as the input data DYI (B) of the second COM driver Y2.
The sixty flip-flops included in the built-in shift register circuit of the first COM driver Y1 for comb-like driving provide outputs O1 to O60 corresponding to two periods of the external signal YSCL as data for scanning each common line (Nos. 1 to 60) based on the input data DYI (A) corresponding to one period of the external signal YSCL. The flip-flops also provide the output data DYO (A) by two periods of the external signal YSCL.
As shown in FIGS. 13 and 15, the output data DYO (A) from the first COM driver Y1 and corresponding to two periods of the external signal YSCL are sent to the second COM driver Y2, so as to serve as the input data DYI (B) of the second COM driver Y2 for comb-like driving. As a result, the sixty flip-flops included in the built-in shift register circuit of the second COM driver Y2 for comb-like driving provide outputs O1′ to O60′ corresponding to four periods of the external signal YSCL as data for scanning each common line (Nos. 61 to 120) based on the input data DYI (B) corresponding to two periods of the external signal YSCL. The flip-flops also provide output data DYO (B) corresponding to four periods of the external signal YSCL.
However, multiple COM drivers for comb-like driving in cascade connection used as described above involve the following problem. The output data DYO (A) from the first COM driver Y1 for comb-like driving are output by two periods of the external signal YSCL, which is a basic clock provided from outside for driving the drivers. This hinders intended operations of the second COM drivers for comb-like driving in cascade connection.
To put it differently, when the first and second COM drivers Y1 and Y2 for comb-like driving in cascade connection scan lines as shown in FIG. 12, the scanning data of two periods output from the built-in flip-flops make the drivers scan every two lines as intended in the upper half of the LCD panel in the same manner as comb-like driving shown in FIG. 11. However, in the lower half the scanning data of four periods output from the built-in flip-flops make the drivers scan two lines simultaneously with a one-line interval. Consequently, two lines are active at a time, which doubles energy consumption of a driver integrated circuit on the whole.
In consideration of the above-mentioned problem, the invention aims to provide a driver for a liquid crystal device that not only can operate the second and following COM drivers normally in using multiple COM drivers for comb-like driving in cascade connection, but also can reduce energy consumption.